groundfloor,
Lots of Operators think they can write up their own SOP`s which is true but comes with some serious repercussions. They have to be approved by "the relevant AUTHORITY" being your CAA FAA JAA - not the manufacturer - there is no Airbus or Boeing Authority.
It seems that the UK CAA disagrees with you - see UK CAA CAP 676, Chapter 3, Step 1:
If the procedure in the AFM requires amendment, this must be carried out by the manufacturer or other organisation approved to make such amendments prior to changing the checklist.